Expression vectors containing a truncated epstein barr nuclear antigen 1 lacking the Gly-Gly-Ala domain for enhanced transient gene expression

ABSTRACT

This invention relates to the unexpected discovery that nucleotide coding sequences coding for a truncated Epstein Barr Nuclear Antigen 1 (EBNA1t) protein (lacking the Gly-Gly-Ala domain), when in cells of mammalian origin, are associated with improved growth and increased transient gene expression when compared with cells expressing a complete EBNA1 coding sequence. The expression of EBNA1t also appear to be more stable over time.

FIELD OF THE INVENTION

This invention relates to new mammalian cells and cell lines, especially CHO and 293 cell lines, which comprise expression vectors encoding truncated EBNA1 genes which enhance transient gene expression. The invention also relates to expression cassettes which include such truncated genes.

BACKGROUND OF THE INVENTION

Mammalian cells are an established expression system in the biotechnology industry for the production of recombinant proteins (r-proteins). In contrast to lower eukaryotes or prokaryotes, mammalian cells provide active r-proteins that possess relevant post-translational modifications. However, in order to obtain sufficient amount of protein for structure/activity analyses or high-throughput screenings, one needs to go through the long and tedious process of stable clone isolation and characterization. Protein production by large-scale transfection is an interesting alternative to the generation of stable clones as it allows the very fast generation of mg to gram quantities of r-protein within few days.

The use of vectors containing the Epstein-Barr virus (EBV) oriP in cell lines stably expressing EBV's EBNA1 protein, such as the HEK293-EBNA1 (293 E) cell line (ATCC#CRL-10852) significantly increases protein yield (Durocher et al., 2002). EBNA1 is a multi-functional protein that have been shown to positively regulate many viral promoters present on plasmid DNA when the oriP is present in cis (Reisman and Sugden, 1986).

The production of secreted r-protein often needs to be performed in serum-free medium in order to facilitate their purification. Adaptation of the 293E cell line to serum-free medium formulations is not straightforward and is rarely successful. To circumvent this problem, the generation of new 293-EBNA1 cell line from a serum-free medium adapted 293 cell line is preferable (Pham et al., 2003; Pham et al., 2005). However, these new cell lines do not always show optimal growth properties or high transfectabilities in serum-free medium. Also, the isolation of new clones stably expressing full-length EBNA1 is difficult as this protein seems to be cytotoxic to the cells.

Preliminary transient gene expression studies with the commercially available 293F cells adapted to the FreeStyle™ medium showed that this cell line has a good potential for the large-scale r-protein production in serum-free medium. Improvement of this cell line by stably expressing a less cytotoxic but functional EBNA1 protein is needed.

Kennedy, G. and Sugden, B. (2003) EBNA-1, a Bifunctional Transcriptional Activator Molecular and Cellular Biology, 23: 6901-6908 disclose that the ability of EBNA1 to activate transcription from both integrated and transfected templates can be inhibited by a derivative of EBNA1 lacking the amino acids required for activation from integrated templates (aa 65-89). We have found, against previous expectations, that truncations of these amino acids from EBNA1-coding nucleotide sequences can enhance transient gene expression in HEK293 cells to a level similar to EBNA1.

SUMMARY OF THE INVENTION

This invention relates to the unexpected discovery that nucleotide coding sequences coding for a truncated Epstein Barr Nuclear Antigen 1 (e.g. EBNA1t) protein (lacking the Gly-Gly-Ala domain), when in cells of mammalian origin, are associated with increased transient gene expression when compared with control cells. In addition, expression of this truncated EBNA1 gene is more stable and expressed at higher levels than expression of the full-length EBNA1 gene. This results in cell lines with better growth properties and with enhanced transient gene expression. Mammalian cell lines in general are contemplated and human embryonic kidney 293 cells, CHO cells and PER-C6™ cells are of particular interest. This invention also relates to a mammalian cell line such as a 293 cell line stably expressing a processed version of EBNA1t (e.g. 293-6E cells) also showing enhanced transient gene expression compared to EBNA1t, EBNA1 and control cell lines.

Preferably the transfected gene expression is performed in a cell line stably expressing truncated EBNA1. Alternatively, the transfected gene expression is associated with a transiently transfected EBNA1 gene. Also, preferably the EBNA1 nucleotide sequence is truncated to lack most of (i.e. more than 50%, preferably more than 75% and, in some embodiments, all) the Gly-Gly-Ala domain. Preferably the nucleotide sequence is less than 70% of a complete EBNA1 coding sequence, especially less than 50% of the complete EBNA1 coding sequence. Alternatively, or as well, one or more of the DNA linking regions LR1 and LR2 can be absent from the truncated sequence. One of the truncated sequences we have used lacks LR1 and we expect that an equivalent sequence lacking LR2 (with or without LR1 present) to serve a similar purpose. The nucleotide sequence can be included in an expression vector, such as a pTT vector or any other vectors containing a complete or partial Epstein Barr Virus (EBV) oriP sequence, allowing expression of the gene.

Stable cell lines including such expression vectors with truncated EBNA1 nucleotide coding sequences comprise an aspect of the invention.

According to one aspect of the invention, we provide new stable serum-free 293F-EBNA1 cell lines, including full-length of truncated versions of EBNA1.

The use of EBNA1t reduces the difficulty of obtaining stable clones (apparent deleterious effects of over-expressing the full-length EBNA1 protein). To our knowledge, no reports describing stable 293-EBNA1t cell lines exist. Also, by isolating and characterizing a stable 293F-EBNA1t cell line (clones 6E), we observed another new further truncated and functional form of EBNA1, of even shorter amino acid sequence length than EBNA1t (location of truncation not yet identified).

According to another aspect of the invention we provide a series of new truncated EBNA1t expressed proteins (including EBNA1c).

The following aspects of the invention are described in detail below.

-   -   1. The new 293FEt cell line, where Et is a truncated version of         the EBNA1 protein e.g. EBNA1t described below and in the         figures.     -   2. The new 293-6E cell line expressing a processed form of         EBNA1t protein     -   3. The new truncated EBNA1 protein, EBNA1c consisting of         LR2+NLS+DBD domains     -   4. Using transient EBNA1 (full-length or truncated) expression         in trans to increase protein production in EBNA1 (full-length or         truncated) and non-EBNA1 cell lines     -   5. The use of an EBNA1t or EBNA1c expression cassette in the pTT         vector or other oriP-containing vectors (expression in cis) to         increase protein production in EBNA1 and non-EBNA1 cells.     -   6. New truncated EBNA1 protein consisting of LR1+NLS+DBD         domains.

The invention further relates to a process for in vitro production of a protein which process comprises:

-   -   (a) transfecting a mammalian cell with an expression vector         coding for said protein, said mammalian cell having been         transfected with a truncated EBNA1 expression vector of the         invention;     -   (b) culturing a transfected cell resulting from (a) to yield         said protein.

BRIEF DESCRIPTION OF THE DRAWINGS

In drawings which illustrate the invention,

FIG. 1 shows transient SEAP expression in 293F cells following co-transfection of various amounts of pTT/EBNA1t vector.

FIG. 2 shows stable or transient EBNA1 constructs expression in 293 cells.

FIG. 3 shows transient GFP expression in various 293F-EBNA1 clones or pools.

FIG. 4 shows a Western Blot analysis of EBNA1 expression in various 293F clones.

FIG. 5 shows transient human placental secreted alkaline phosphatase (SEAP) expression in various EBNA1 clones.

FIG. 6 shows the growth of various 293F-EBNA1 clones following transfection (hpt=hours post-transfection).

FIG. 7 shows the growth curve of 293-6E cells compared to 293F cell in 125 ml shaker flasks.

FIG. 8 shows the amino acid sequence of EBNA1 (SEQ ID NO: 1) with various parts of the sequence identified in the Figure.

FIG. 9 shows the amino acid sequence of full-length EBNA1 protein (SEQ ID NO: 1) and EBNA1t (underline) (SEQ ID NO: 2) and EBNA1c (bold) (SEQ ID NO: 3) truncated versions. The first amino acid of the new EBNA1c protein is a methionine (as indicated above the glycine residue).

FIG. 10 shows the schematic structure of various EBNA1 constructs.

FIG. 11 (A-C) shows DNA sequence of full-length EBNA1 (SEQ ID NO: 4), and truncated EBNA1 (EBNA1t (SEQ ID NO: 5) and EBNA1c (SEQ ID NO: 6)).

FIG. 12 shows transient EBNA1t and EBNA1c expression in 293F cells compared to 293F or 293-6E cells.

FIG. 13 shows the effect of co-expressing EBNA1t or EBNA1c on transient SEAP expression in 293F cells.

FIG. 14 shows examples of proteins transiently expressed in 293-6E cells.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to nucleotide coding sequences coding for a truncated Epstein Barr Nuclear Antigen 1 (EBNA1) protein which, when in cells of a mammalian cell line, are associated with increased transfected gene expression when compared with cells of a control cell line comprising a complete EBNA1 coding sequence. By “truncated” we mean a sequence which is less than the full EBNA1 nucleotide sequence. As shown in FIGS. 8 and 10 there are identified components of the full EBNA1 sequence. These include DNA Linking Regions 1 and 2, a Transcription activation domain, a Nuclear Localization Signal and a DNA Binding and Dimerization region. Truncated sequences of the invention preferably contain the DNA Binding and Dimerization region along with the Nuclear Localization Signal and one or more DNA Linking Regions. FIG. 1 shows that transient SEAP expression can be increased significantly by co-expression of EBNA1t protein. Similar increase can be observed using full length EBNA1 protein (not shown). This Figure also shows that transient SEAP expression does increase by augmenting EBNA1t expression. However, it seems that over expressing full-length EBNA1 is difficult to achieve in mammalian cells. This is illustrated in FIG. 2 where stable expression of full-length EBNA1 in the commercially available cell line HEK293-EBNA1 (formerly available at Invitrogen or available at ATCC #CRL-10852) or in our best SFE clone (SFE41; (Pham et al., 2003)) is significantly lower than in 293FEt bulks (lanes 6 and 7) or 293-6E cells (lanes 8 and 9). In addition, expression of full-length EBNA1 in 293F cells (bulk) in also very low (lanes 4 and 5). Note that while truncated forms of EBNA1 increases with time in these bulks (lanes 6 vs 7 and lanes 8 vs 9), expression of full length EBNA1 drops with time (lanes 4 vs 5), indicating that overexpression of full-length EBNA1 may have negative effects on cell physiology. Unexpectedly, a major and smaller form of EBNA1t was observed in clone 6E (lanes 8 and 9).

EBNAlt was amplified by PCR using forward (ACGGAATTCGCCGCCACCATGTCTGACGAGGGGCCA) (SEQ ID NO:7) and reverse (GAGGAAGGGCAGGAGTGAGAATTCCCT) (SEQ ID NO:8) primers and cloned at the EcoRI site of pIRES-Neo vector (Clontech). We made the 293FEt cell line (including the 293-6E clone) following transfection of 293F cells with the pIRES-EBNAlt-Neo vector and selection with 25 μg/ml geneticin. Stable clones were isolated by limiting dilution and clones selected based on EBNA1 expression using the rat monoclonal antibody 1H4 (Grasser et al., 1994). The 293-6E clone was deposited on Mar. 15, 2005, as a Budapest Treaty deposit in the International Depositary Authority of Canada, National Microbiology Laboratory, Health Canada, 1015 Arlington Street, Winnipeg, Manitoba Canada R3E 3R2, as accession number 150305-01.

FIG. 1 shows that expressing EBNA1t increases transient SEAP expression in a dose-dependent manner. 293F cells were transfected with 100% pTT-SEAP vector (CTRL) or with mixtures of 99 to 60% pTT-SEAP and 1 to 40% of pTT-EBNA1t respectively. With 60% pTT-SEAP and 40% pTT-EBNA1t, the expression level of SEAP was increased by 3-fold over control.

FIG. 2 shows EBNA1 expression levels in various stable HEK293 cell lines or following transient transfection. Stable expression of full-length EBNA1 in HEK293-EBNA1 cell line (lane 1) and 293-SFE cell line (lane 2). Lane 3: 293F cells (no EBNA1 expression). Expression of full-length EBNA1 in 293F cells following transfection and G418 selection for 2 months (lane 4) and 4 months (lane 5). Note that expression of full-length EBNA1 decreases with time in this non-clonal cell population. Expression of truncated EBNA1 (EBNA1t) in 293F cells following transfection and G418 selection for 2 months (lane 6) and 4 months (lane 7). Note that expression of EBNA1t increases with time in this non-clonal cell population. Expression of the new form of EBNA1t in clone 6E derived from 293F-EBNA1t after 4 months in culture in the presence of G418 and 1% serum (lane 8) or G418 in serum-free medium (lane 9). Transient expression of full-length EBNA1 (lane 10) or EBNA1t (lane 11) in 293F cells.

The precise nature of the new EBNA1t protein remains to be solved. Detection of EBNA1 was performed using a rat monoclonal antibody (clone 1H4). The two bands seen at Mr 200 and above are not-specific.

FIG. 3 shows transient GFP expression in various EBNA1 cell lines. Cells (cultured for 3 months under G418 selection following transfection) were transfected with pTT-GFP and GFP expression was measured 3 days later by flow cytometry. The 293F-EBNA1t clone 6E shows the highest GFP expression. Transfection efficiency was between 40% and 65% for all clones.

FIG. 4 shows EBNA1 expression levels in various 293 clones. All clones were cultured in the presence of 25 μg/ml geneticin. Expression of EBNA1 in clone 6A can be detected with longer exposure time.

FIG. 5 shows that when various 293F-EBNA1 stable clones were transfected with pTT/SEAP, the clones expressing truncated EBNA1 coding sequences showed enhanced SEAP expression when measured 5 days later (clones 6E, 11 and 13) when compared to clones expressing the full length (clones 1A and 2B) or another uncharacterized truncated form of EBNA1 (clone 6A). In the context of this invention, SEAP is an example of a recombinant protein. Genetic material coding for a protein or polypeptide of choice can be used in place of SEAP coding sequences and, indeed, this is an aim of this invention (see FIG. 14 for additional examples).

FIG. 6 shows that cell growth and viability does not appear to be affected when truncated EBNA1 nucleotide sequences are stably overexpressed. Cells were fed with 0.5% TN1 24 hpt (Pham et al., 2005) and counted 6 days after transfection.

FIG. 7 shows the growth characteristic of the 293FEt-clone 6E (lower panel) compared to the parental 293F cell line (upper panel). Maximum viable cell density is about 3.5×10⁶ cells/ml for the clone 6E compared to 4.2×10⁶ cells/ml for the 293F cell line.

FIG. 8, FIG. 9, FIG. 10 and FIG. 11 are best reviewed together. They show the amino acid sequence and schematic structure of EBNA1 constructs and the relationship to the EBNA1 DNA sequences (FIGS. 11A-C).

FIG. 8 shows the EBNA1 full length protein (641 aa, 56.4 kDa) (Accession number: NC_(—)001345) with its main features. FIG. 9 highlights the differences between EBNA1, EBNA1t and EBNA1c and the amino acid level. EBNA1t truncated protein (underline: 417 aa, 42.5 kDa) and EBNA1c further truncated protein (bold: 306 aa, 32.5 kDa). The first amino acid of the new EBNA1c protein is a Methionine (as indicated above the Glycine residue).

FIG. 12 contrasts transient expression of two truncated EBNA1 constructs with 293F and 293-6E cells. Cells were transfected with pTT/EBNA1t or pTT/EBNA1c vectors and EBNA1 expression was detected 3 days later by Western blot. Non-transfected 293F cells and 293-6E cells are also shown as controls.

FIG. 13 shows 293F cells co-transfected with pTT/SEAP and pTT/EBNA1 constructs. 293F cells were co-transfected with a mixture of 50% pTT-SEAP vector with pTT/EBNA1t, 50% pTT/EBNA1c, or 50% salmon sperm DNA (stuffer DNA). SEAP expression was measured 5 days later.

FIG. 14 shows examples of proteins transiently expressed in 293-6E cells. 293-6E cells were transfected with pTT vectors encoding various secreted proteins and culture medium (20 microliters) was harvested 5 days after transfection and analyzed by SDS-PAGE and Coomassie staining.

REFERENCE LIST

-   Durocher, Y., Perret, S., and Kamen, A., 2002. High-level and     high-throughput recombinant protein production by transient     transfection of suspension-growing human 293-EBNA1 cells. Nucleic     Acids Res. 30, E9. -   Grasser, F. A., Murray, P. G., Kremmer, E., Klein, K., Remberger,     K., Feiden, W., Reynolds, G., Niedobitek, G., Young, L. S., and     Mueller-Lantzsch, N., 1994. Monoclonal antibodies directed against     the Epstein-Barr virus-encoded nuclear antigen 1 (EBNA1):     immunohistologic detection of EBNA1 in the malignant cells of     Hodgkin's disease. Blood 84, 3792-3798. -   Pham, P. L., Perret, S., Cass, B., Carpentier, E., St-Laurent, G.,     Bisson, L., Kamen, A., and Durocher, Y., 2005. Transient gene     expression in HEK293 cells: peptone addition posttransfection     improves recombinant protein synthesis. Biotechnol. Bioeng. 90,     332-344. -   Pham, P. L., Perret, S., Doan, H. C., Cass, B., St-Laurent, G.,     Kamen, A., and Durocher, Y., 2003. Large-scale transient     transfection of serum-free suspension-growing HEK293 EBNA1 cells:     peptone additives improve cell growth and transfection efficiency.     Biotechnol. Bioeng. 84, 332-342. -   Reisman, D. and Sugden, B., 1986. trans activation of an     Epstein-Barr viral transcriptional enhancer by the Epstein-Barr     viral nuclear antigen 1. Mol. Cell Biol. 6, 3838-3846.

The above-described embodiments of the present invention are intended to be examples only. Alterations, modifications and variations may be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined solely by the claims appended hereto. 

The invention claimed is:
 1. A mammalian cell line that stably expresses a truncated Epstein Barr Nuclear Antigen 1 (EBNA1) protein, which cell line is the 293-6E cell line deposited at the International Depositary Authority of Canada under accession number 150305-01.
 2. A process for transient in vitro production of a recombinant protein, which process comprises: (a) transfecting a mammalian cell line in accordance with claim 1 with an expression vector coding for a recombinant protein; and (b) culturing the transfected cell resulting from (a) to yield said recombinant protein.
 3. A process in accordance with claim 2, wherein said expression vector further includes a complete or partial Epstein Barr Virus (EBV) oriP sequence. 